Green’s Dictionary of Slang

astounding, amazing, remarkable, orig. in the context of hallucinogenic drug use; thus mind-blowingly adv.

1968		Times 4 May 21: The poet celebrates the mindblowing effects of LSD.
1971		J. Mandelkau Buttons 87: The Fisherman’s Wharf dance for Geno Haniki’s widow was a mind-blowing event.
1992		D. Jarman diary 8 Nov. Smiling in Slow Motion (2000) 252: Colin’s report on Kenneth Branagh’s new film was even worse – mind-blowingly trivial.
1999		Eve. Standard 4 June 54: Egypt may be magnificent on land, but fathoms down it’s mind-blowing.
2000		Indep. on Sun. Culture 9 Jan. 8: I thought it was quite the most mind-blowing experience I’ve had.